fishnet v2.2.1

Changes:

• Update Stockfish for multiple speedups and Elo gainers: official-stockfish/Stockfish@51deae8...6dddcec.
• Update and deduplicate dependencies (including tokio 1.0).
• Some future proofing: Send OS and architecture in User-Agent header, implement proposed new key validation, accept up to 24 byte batch ids, do not send progress reports for full depth/multi-pv matrix analysis.
• Build aarch64-apple-darwin releases on stable Rust.

Bugfixes:

• Show reason if server rejects the client.
• Avoid panic when logging to closed stdout, fallback to stderr.
• Do not ignore error when setpgid() for Stockfish process fails.